1. Field of the Invention
The present invention relates to dielectric ceramics, and particularly to dielectric ceramics that can be fired together with copper or silver and that can be used for electric parts (e.g., wiring board, dielectric resonator, LC filter, capacitor, dielectric waveguide, dielectric antenna, etc.) adapted for use at high frequencies such as micro waves and millimeter waves.
2. Description of the Prior Art
In the age of highly sophisticated information in recent years, it is a tendency to transmit information at higher speeds at higher frequencies. In the new media such as mobile wireless systems like automotive telephones and personal wireless systems, or satellite broadcast systems, satellite communication systems, CATV, etc., attempts have been made to use higher frequencies accompanied by a strong demand for applying higher frequencies even for the existing circuit elements for microwave use such as dielectric resonators.
In the circuit elements for microwave use, it is necessary to lower dielectric loss tangent. For this purpose, it is desired to use a dielectric having a small dielectric loss tangent and to use a low-resistance metal such as copper or silver as a conductor for forming circuits.
In order to meet the above-mentioned demand for decreasing the dielectric loss tangent, Japanese Laid-Open Patent Publication No. 225825/1993 discloses a circuit board made of a dielectric ceramic composition containing a composite perovskite-type compound.
However, the circuit board of the above-mentioned prior art is produced through firing at a temperature as high as from 1300 to 1600.degree. C. leaving a problem in that it is not allowed to form a conductor circuit through the simultaneous firing with a low-resistance metal such as copper or silver. Furthermore, a circuit board has heretofore been known by using an insulating board made of alumina. In this case, too, the firing must be conducted at a high temperature making it difficult to form a conductor circuit by the simultaneous firing with copper or silver. Namely, these circuit boards have a fatal defect in that it is not allowed to form a multiplicity of layers by using a low-resistance metal such as copper or silver or it is not allowed to fabricate fine wirings.
It can be further contrived to use a glass ceramic to avoid the above-mentioned problems. The glass ceramic material can be fired at a temperature of not higher than 1000.degree. C. and can, hence, be fired together with a low-resistance metal such as copper or silver, making it possible to realize a multi-layer structure. However, most of the conventional glass ceramic materials have dielectric loss tangent which is as large as 30.times.10.sup.-4 in a microwave region of 10 GHz, and are not capable of decreasing the dielectric loss tangent as demanded by the devices used at high frequencies. Moreover, these glass ceramic materials must contain a glass component in an amount of at least not smaller than 30% by weight, so that they can be fired at a temperature of not higher than 1000.degree. C. As a result, properties of the obtained ceramics vary to a large extent depending upon the properties of the glass, suppressing excellent properties of the filler components used in combination with the glass component.